This invention relates to the distribution of air or other gas within a body of liquid, and more particularly, within a gravity filter. The use of air to backwash gravity filters is becoming more common in the water treatment industry in the United States. On the other hand, air backwashing has been used in Europe for several decades. Based on the experiences of these European facilities, water treatment engineers have found that a filter bed can be more effectively cleaned through the use of air, rather than with water alone.
A typical backwash sequence using only water consists of a high velocity, reverse flow through the filter bed for a relatively long period of time. Typical backwash flow rates are in the range of 15-20 GPM/ft.sup.2 (gallons per minute per square foot). This compares to a normal filtering flow rate (downward) of 3-5 GPM/ft.sup.2. This water backwash, depending upon the solids loading in the raw water, is done once every one or two days and lasts from 10-20 minutes.
Air backwashing sequences typically follow two basic patterns. The first involves a cycle of air backwash at a flow rate of around 2-4 SCFM/ft.sup.2 (standard cubic feet per minute per square foot) for about five minutes. This backwash using only air is then followed by a shortened water backwash for about five minutes at the aforementioned flow rate of 15-20 GPM/ft.sup.2.
A second approach is to include a simultaneous air/water cycle between the separate air and water cycles listed above. Rates for these simultaneous flows are typically 2-4 SCFM/ft.sup.2 and 3-5 GPM/ft.sup.2. Duration of this simultaneous flow cycle is around five minutes. When this approach is used, the duration of the final cycle using only water can be further shortened to around five minutes.
A calculation of the total water used for backwash shows that, with the aforementioned second approach, water usage can be reduced from 150-400 gallons/ft.sup.2 using only a water backwash cycle, to about 90-125 gallons/ft.sup.2, a significant decrease. Additionally, the filter bed is usually better cleaned with the incorporation of an air backwash, resulting in longer filter runs between backwashes. These longer runs further reduce the net usage of water for backwashing. There is an obvious economic advantage to this process and, consequently, it is becoming more common.
The common method of underdrain design in Europe involves the use of hundreds or thousands of individual, direct retention filter nozzles placed in a false floor. This false floor is generally about 12" above the true filter floor and provides a chamber for flows to be collected during filtering and distributed during backwash. Distribution of water in such a system is accomplished relatively easily by limiting the flow area through the connection point between each nozzle and the floor. This limited flow area introduces a pressure drop which tends to make the water flow towards all of the nozzles rather than be concentrated in just some of the nozzles. However, if air is also to be distributed, additional design problems are introduced. The designers of these systems which utilize air during backwash, realized that air, because it is much less dense than water, would see very little pressure drop through a nozzle connection point designed to distribute water. Hence, air introduced into such a system would tend to all exit through the first few nozzles that it encountered. If, on the other hand, one sized the nozzle connection openings small enough to provide an even distribution of air, then the pressure drop encountered during water flow would be unacceptably high. A means of "pre-distributing" the air flow was needed.
This pre-distribution is typically accomplished through the incorporation of "drop-tubes" in the connection point of each nozzle. Such tubes are typically about 6" long and are open on the lower end. Their diameter is the same as that of the connection point. These drop-tubes have either a slot or a series of small holes in the wall of the tube, forming side openings which do not extend all the way up to the nozzle, but start at a defined elevation below the nozzle. This placement allows the formation of an air "plenum" which distributes air throughout the entire underside of the false floor before air can begin to exit any of the side holes in the drop tube. As more air is introduced into the system, the elevation of the air/water boundary continues to decrease until a balance is achieved between the total air flow out of all of the drop-tube orifices and the air flow into the system. By using tubes of the type described, simultaneous air/water distribution is possible since water can flow through the open end of the air tubes without disrupting the plenum.
The disadvantages of the type of system just described are its expense, the need to provide a structurally sound false floor, the fragility of the individual nozzles which are usually plastic, and the difficulty of retrofitting such a system into existing header/lateral type filters.
An alternate method of incorporating air into a filter is to install an independent air distribution system, such as a drilled pipe header lateral. However, such an approach is quite expensive and also requires the placement of the air distributors above the graded support gravel common to many filters. If such a distributor were placed below the gravel, it would normally cause uplifting and disruption of the gravel layers, resulting in a shortened filter bed life.
Yet another method of incorporating air distribution is one in which a separate air distributor section is incorporated within an underdrain "lateral" which takes the form of a block. Since the design is relatively complex, the cost of such an underdrain is relatively high. Also, such a design requires the use of graded gravel above the block. Although the stability of the gravel can be fairly well controlled by maintaining a limited air flow rate, gravel disruption is still a problem with this design.
One example of a patented prior art construction is Parmellee U.S. Pat. No. 801,810 which has small upper apertures on a single upper level to distribute air into a bed of coarse gravel, and large lower apertures on a single level to distribute water. Sasano et al U.S. Pat. No. 4,214,992 is similar to Parmalee in that it has air orifices at a single level and does not appear to address simultaneous air/water backwash. Davis et al U.S. Pat. No. 4,707,257 shows air and water orifices in the same chamber and at the same elevation, thus making the unit sensitive to levelness. It incorporates dual plenums, with one air plenum being located under a false floor and the other under a flat distribution plate containing small segments of screen for supporting the filter media. Evans et al U.S. Pat. No. 5,015,383, and co-pending Division Ser. No. 698.900, filed May 13, 1991, which are assigned to a common assignee, and are hereby incorporated by reference, show scallop-shaped underdrain laterals in FIGS. 7 and 15-17 which include internal distribution members. The perforations which are disclosed in the internal distribution members are sized to accommodate water flows, and thus would not distribute air uniformly if the lateral were other than perfectly level. For example, the distribution pipes disclosed in FIGS. 15 and 16 have openings only in their lower surface. This arrangement would produce a plenum if air was introduced, but the air would all tend to flow out of the holes at the higher end of the lateral.